A boiler that includes a housing is disclosed. The housing houses a combustion chamber, a heat exchanger system, a heat flow path, an isolating member, and a movable access panel. The heat flow path thermally couples the combustion chamber and the heat exchanger system. The isolating member at least partially separates the combustion chamber from the heat exchanger system. A fluid jacket is operable to thermally couple fluid disposable within the fluid jacket about a fluid side area of heat exchangers of the heat exchanger system. The movable access panel is positioned about or coupled to an exterior wall of the housing. When the movable access panel is moved to an open position, a user is provided access to a gas side area of the heat exchangers for servicing or cleaning of the heat exchangers from an exterior environment of the boiler.